This is for boards only right?  I haven't seen any reports of thumb failures, so I'm assuming this is a component unique to the boards.  Testing out the thumbs now, three of the four are performing perfectly (slightly overclocked and getting 2.6-4GH/s hash rates each).  One thumb was throwing hardware errors out of the box and always underperforms - once the voltage was set to 950mV, the errors went away (until I overclocked the other boards).